Biomedical subjects

Wladimir Urbach

Publications and source records attributed to Wladimir Urbach.

6 recordsLinked to original sources

PubMed2006

Effect of surfactant conformation on the structures of small size nonionic reverse micelles: a molecular dynamics simulation study.

We used constant pressure (P=0.1 MPa) and temperature (T=298 K) molecular dynamics simulations to study the structures and dynamics of small size reverse micelles (RMs) with poly(ethylene glycol) alkyl ether (CmEn) surfactants. The water-to-surfactant molar ratio was 3, with decane as the apolar solvent. We focused on the effect of the two possible imposed conformations (trans vs gauche) for the surfactant headgroups on RMs structures and water dynamics. For this purpose, we built up two RMs, which only differ by their surfactant headgroup conformations. The results obtained for the two RMs were compared to what is known in the literature. Here, we show that the surfactant headgroup conformation affects mainly the water-related properties such as the water core size, the area per surfactant headgroup, the headgroup hydration, and the water core translational diffusion. The properties computed for the RM with the surfactant in trans conformation fit better with the experimental data than the gauche conformation. We further show that the surfactant hydrophilic headgroup plays a crucial role in the micellar structures, favors the entrapment of the micellar water, and reduces strongly their diffusion compared to the bulk water.

Computer Simulation↗

PubMed2006

Structure, stability, and hydration of a polypeptide in AOT reverse micelles.

In this communication, we provide theoretical evidence that the folded structure of a simple peptide, alanine zwitterionic octapeptide, or A8, unstable in solution, becomes stable in a reverse micelle (RM) of appropriate size. Our molecular dynamics simulations were carried out for realistic models of sodium 2-ethylhexylsulfosuccinate RM in isooctane, simulated for an extended period of time. For the RM of the smaller size, we find that a helical structure is stable for the whole length of the simulation. On the contrary, the peptide very quickly takes an extended structure in larger micelles.

Dioctyl Sulfosuccinic Acid↗

PubMed2005

Effect of a neutral water-soluble polymer on the lamellar phase of a zwitterionic surfactant system.

We have studied the effect of adding a water-soluble polymers (PEG) to the lamellar phases of the ternary system tetradecyldimethylaminoxide (C14DMAO)-hexanol-water. The results of Freeze-Fracture Electron Microscopy (FFEM) and Small Angle X-ray Scattering (SAXS) experiments show that the addition of the polymer induces the spontaneous formation of highly monodisperse multilayered vesicles above a threshold polymer concentration.

Journal Article↗

PubMed2005

Bounded step superdiffusion in an oriented hexagonal phase.

Fluorescence recovery after pattern photobleaching is used to measure the self-diffusion of surfactant molecules, along cylinders and perpendicular to their main axis in an oriented hexagonal lyotropic phase. Unexpectedly, while the motion along cylinders is diffusive, a superdiffusive behavior is observed in the direction perpendicular to the cylinder axis. Moreover, varying the lattice parameter, we found that the perpendicular diffusion time is governed only by the number of cylinders to cross, providing experimental evidence for superdiffusion with a bounded step length.

Journal Article↗

PubMed2003

Compressibility of nano inclusions in complex fluids by ultrasound velocity measurements.

We present a high precision ultrasonic velocimeter for a small volume sample (1 cm3) for a path length of 1 cm achieved. The method used is based on the time of flight measurement with an original signal processing technique: the barycenter method. With our system, we have measured the sound velocity with an accuracy of 10(-5). The detection of a difference in velocity between two liquids of about 2 cm/s is achieved. The compressibility of the reference liquid can then be deduced with an accuracy better than 0.2%. Using this custom-made system, we have studied and characterized complex fluids, systems biomimetic of biological membranes, as well as proteins included in nanometric water droplets. Under these experimental conditions, we have reached the value of protein compressibilities with an accuracy better than 10%.

Elasticity↗

PubMed2002

Self-diffusion and collective diffusion in a model viscoelastic system.

We use dynamic light scattering (DLS) and fluorescence recovery after pattern photobleaching (FRAPP) to investigate the dynamics of a model transient network made of an oil-in-water droplet microemulsion to which small amounts of a telechelic polymer are added. The DLS correlation functions exhibit three relaxation modes. The two first modes can be interpreted quantitatively in the frame of the classical De Gennes-Brochard theory of DLS in viscoelastic system. The third, slower mode is diffusive and arises from the ternary character (droplets, polymers, and water) of the system. By contrast, the pattern relaxation in FRAPP exhibits a single-, slow-exponential decay with a characteristic time proportional to the squared inverse scattering vector: the corresponding self-diffusion coefficient of the droplets is found to be close to the diffusion coefficient characterizing the third mode in DLS. We interpret these results in terms of the coupled relaxation of the concentration fluctuations of the polymers and the droplets.

Journal Article↗